But for future reference to people who find this in the future, this is already implemented.
The apt source packages are integral to our debian build process and are hosted in the same repositories as the binaries. If you find a mirror which is missing them please reach out or file a ticket with that mirror maintainer. (However some administrators may choose to skip them for bandwidth/storage reasons.)
We don’t enable them by default in our instructions to keep things simpler and avoid confusion for new users. For power users who are interested in using the source packages they can enable them manually. The link from @gavanderhoorn is the best documentation of how to enable the download of source packages on a machine.
Ok, thanks all. I was under the false assumption that they just didn’t exist (which is why I erroneously posted on discourse instead of answers) due to the fact that:
I could not find any documentation about it
I was trying to use apt-src install ros-humble-$PACKAGE which doesn’t work. So that particular tool must have some assumptions about source repositories that is not true for the way that the ROS repositories are setup.
But yes, apt-get source does work, including for ROS 2 packages.